In recent years, the types of communication of phones have become more diverse. In addition to the conventional speech communication by line switching, phones capable of performing packet communications utilizing an IP (internet protocol) network are widely utilized. For example, “chat mail” where mobile phones trade text in real time for conversion is known (see Patent Document 1). Chat mail is one of the modes of communication called “group communication” where members registered in advance form a group and trade text and other data in real time in the group.
Further, as one of the types of group communications attracting attention recently, the one called PTT (push to talk) or PoC (push to talk over cellular) is known. In the PTT, a plurality of communication terminals forming a group engage in speech conversation with each other in real time by VoIP (voice over Internet protocol) (see Patent Document 2).
When a plurality of communication terminals capable of group communication by PTT try to start group communication, a representative single communication terminal among the plurality of communication terminals performs processing for starting the group communication. Then, that representative single communication terminal generates a group communication start notification and sends the notification to the other communication terminals registered in that communication terminal. The “group communication start notification” is a notification whereby that representative single communication terminal inquiries to the other communication terminals about their participation in group communication.
The owners of the other communication terminals receiving the group communication start notification can determine whether or not they will participate in the group communication by using, for example, data included in the group communication start notification transmitted from the owner of representative single communication terminal as a judgment standard. Once the PTT group communication is started, the owner of any communication terminal desiring to say something can depress a predetermined button (PTT button) provided at the communication terminal to transmit a speech request of the owner to a server apparatus. The server apparatus receives the speech requests from the communication terminals and gives a right of speech to a single communication terminal. The communication terminal given the right of speech transmits speech data of the owner to the server apparatus, whereupon the server apparatus transmits that speech data to the other communication terminals all together. Due to this, speech conversation in real time is carried out by a plurality of communication terminals forming a group.    Patent Document 1: Japanese Patent Publication (A) No. 2003-174520    Patent Document 2: Japanese Patent Publication (A) No. 2005-522067